Reading on electronic devices before bedtime can disrupt the body’s circadian clock, making it harder to fall asleep and become alert in the morning, according to a study from Brigham and Women’s Hospital in Boston.

Sleep researchers have long warned that watching TV, working on a computer and other activities involving artificial light in the evening can disrupt sleep patterns. But the type of short-wavelength enriched light, also known as blue light, that many backlit devices emit is especially powerful at suppressing the...